As is well known, a circuit breaker is an automatically operated electro-mechanical device designed to protect a conductor from damage caused by an overload or a short circuit. Circuit breakers may also be utilized to protect loads. A circuit breaker may be tripped by an overload or short circuit, which causes an interruption of power to the load. A circuit breaker can be reset (either manually or automatically) to resume current flow to the load. One application of circuit breakers is to protect motors as part of a motor control center (“MCC”). A typical MCC includes a temperature triggered overload relay, a contactor and a motor circuit protector (“MCP”). The MCP is a specialized circuit breaker that provides instantaneous protection against instantaneous short-circuit events. These motor circuit protector devices must meet National Electric Code (“NEC”) requirements when installed as part of a UL-listed MCC to provide instantaneous short-circuit protection.
Mechanical circuit breakers energize an electro-magnetic device such as a solenoid to trip instantaneously in response to a rapid surge in current such as a short circuit. Existing MCPs protect only a limited range of motors, but should avoid tripping in response to in-rush motor currents that occur during motor start-up while tripping on a range of fault currents including instantaneous short-circuit currents. In order to provide protection for a full range of motors with different current ratings, different MCP circuit breakers that match the operating parameters of the particular motor must be designed for each current rating. Each MCP circuit breaker is designed with specific trip point settings for a given current rating. Thus, many circuit breaker models must be offered to cover a full range of currents.
Currently calibration for mechanical MCPs is performed mechanically by adjusting a screw that adjusts the trip level of the breaker by changing the position of a cross bar until the output matches a test value. This method has the disadvantage of having to take time to measure a test value, adjust the screw, and secure the mechanism for the production unit. These steps add time and expense to production. Such calibration may also result in drifting over time.
Existing calibration methods are part of the manufacturing process and are not incorporated into the product design process. What is needed, therefore, is a process to calibrate the signal chain of a motor circuit protector as part of the design process. Another need is to provide a calibration process to use the saturation region of current transformers to increase the operating parameters of a circuit breaker. There is also a need for a calibration process that may be adjusted via programming without altering the basic test process.